Samples will be collected for inhibitor screening and assay. These will be aliquoted ant stored at -70 degrees C. A minimum of 10% of all hemophilic samples will be forwarded to the reference center for confirmation of presence or absence of inhibitor. For patients with known inhibitor aliquots will be forwarded to the reference center twice yearly as a minimum and following any noteworthy therapeutic or clinical event as well. Patients with high titer inhibitor or significant changes in titer will also have samples forwarded. While monthly samples will be sought, it must be noted that this may not be realistic in some cases due to problems with patient compliance. It may be possible for a member of the staff to make home visits.